1. Field of the Invention.
The subject invention relates to a high frequency common mode ferrite bead for surface mounting on a circuit board.
2. Description of the Prior Art.
Ferrites are widely used as attenuators or suppressors of unwanted signals in electrical circuits. When the problem of EMI/RFI arises in a circuit, the first step is to isolate the source. The second step is to diminish its output to an acceptable level without destroying the necessary information being transmitted at other frequencies.
The prior art has included ferrite suppressors that are inserted into a circuit containing useful signals as well as noise or interference. The desired signals will pass through while the undesired energy will be attenuated. This is accomplished through the impedance of the ferrite which is frequency sensitive. Direct and low frequency currents see only the conductor, and are unimpeded. The high frequency energy couples with the ferrite and impedance is developed which has inductive and resistive components. When a line is passed through a ferrite core, low frequency energy is transmitted with little loss, whereas higher frequency energy encounters the inductive reactance caused by the real part of the complex permeability of the ferrite. The inductive reactance reduces the conducted EMI current and introduces an insertion loss. At still higher frequencies, where the real part of the permeability of the ferrite material decreases and the inductive reactance falls, the lossy characteristics dominate and the resistive componet of the impedance assumes the main role of providing the insertion loss for dissipating the unwanted energy.
Ferrite suppressors have taken many forms in the prior art. A significant number of very desirable ferrite suppressors are sold by Fair-Rite Products Corp., the assignee of the subject application. For example, Fair-Rite part #2744041447 is identified generally by the numeral 10 in FIGS. 1 and 2. The prior art ferrite surface mount bead 10 of FIGS. 1 and 2 includes a generally rectangular ferrite body 12 having top and bottom surfaces 14 and 16, first and second end surfaces 18 and 20 and first and second side surfaces 22 and 24. Two rectangular slots 26 and 28 extend entirely through the ferrite body 14 from the first end 18 to the second end 20 at a position midway between the top and bottom surfaces 14 and 16 and equally spaced from the respective side surfaces 22 and 24. These slots are connected by a narrower slot that is designed to limit the sidewise movement of the conductive strips 30 and 32 that pass through the slots 26 and 28 respectively. Used as a common mode choke, the source and return electrical currents are conducted in opposite directions on these strips. Any direct or low frequency current, which would be equal, create magnetic fields that are equal and opposite, and therefore cancel, resulting in a net zero magnetic field. Any common mode, meaning in-phase signals travelling on both conductors, will be attenuated by the characteristics of the ferrite since the magnetic fields created by the signal would not cancel. The ends of the respective conductive strips 30 and 32 are folded around the respective end faces 18 and 20 of the ferrite body 14 and into abutting face-to-face engagement with the bottom surface 16 thereof. These end faces may then be electrically and mechanically mounted to conductive pads on the surface of a circuit board. Common-mode surface mount beads as shown in FIGS. 1 and 2 have proved to be very effective.
Fair-Rite Products Corp. also sells a ferrite bead identified as part #2943666681 and indicated generally by the numeral 34 in FIG. 3. The bead 34 includes a cylindrical ferrite body 36 having a total of six apertures passing therethrough substantially parallel to the axis of the cylinder. Two conductive wires are threaded through the apertures such that each wire passes through three of the six apertures. In particular, each wire will enter the ferrite core at one end, will pass entirely to the opposed end and reverse directions. The wire will reverse directions again at the original end and will continue back to the opposed end. The wires from the suppressor shown in FIG. 3 will be bent away from an axial direction and down toward a surface board where the wires may be mounted in conductive through holes or may otherwise soldered into connection with conductive regions on a circuit board. The prior art bead 34 is not designed for surface mount applications.
Another prior art ferrite bead is shown in U.S. Pat. No. 5,455,552 which issued to Metsler on Oct. 3, 1995 and is assigned to Steward, Inc. The bead shown in U.S. Pat. No. 5,455,552 includes a generally rectangular ferrite body with two parallel slots extending therethrough. Two conductors of generally U-shape are arranged parallel to one another such that one leg of each conductor passes through each of the two slots in the ferrite body. These conductors continue through the ferrite body and are mounted in conductive through holes of a circuit board. However, when employed as a common mode choke, the magnetic fields created are complex, do not completely cancel, and the close proximity of the conductors to each along with the dielectric constant of the ferrite material that is greater than that of air that results in severe degradation of the attenuative characteristics of the device above about 100 MHz.
It is an object of the subject invention to provide a ferrite common mode bead intended for surface mount applications and exhibiting very high impedance over a very wide frequency range.